1. Field of the Invention
The present invention relates to a system and method for controlled delivery of a gas from a liquefied state. In particular, the invention relates to a system and method for delivery of a gas from a bulk source.
2. Description of Related Art
In the semiconductor manufacturing industry, high purity gases stored in bulk delivery vessels are supplied to process tools for carrying out various semiconductor fabrication processes. Examples of such processes include diffusion, chemical vapor deposition (CVD), etching, sputtering and ion implantation.
Of the numerous gases utilized in the semiconductor manufacturing processes, many are stored in bulk delivery vessels in a liquefied state. A partial list of chemicals stored in this manner, and the pressures under which they are typically stored, is provided below in Table 1:
The primary purpose of the bulk delivery vessel and system is to store the above-listed electronic specialty gases (ESG) and provide a safe vehicle for delivering gas from the vessel to the process tool.
In the manufacture of integrated circuits it is imperative that the electronic specialty gases employed be delivered to the point of use in ultra-high purity form. Ultra-high purity is herein defined in terms of impurity concentrations of less than 100 ppb (parts per billion) for any volatile molecule, and in particular, particulate concentration of a size larger than 0.3 micrometers at less than 1/liter of gas under normal conditions and metallic impurities at less than 1 ppb (parts per billion in atomic units) per element.
In typical gas delivery systems, such as the one disclosed in U.S. Pat. No. 5,673,562 to Friedt, electronic specialty gases are conveyed to the point of use by application of the principle of evaporation thermodynamics. The gas contained in the bulk delivery vessel is maintained in a gas-liquid phase equilibrium and the ultra-pure vapor formed at the upper region of the vessel is conveyed to the point of use under the its own vapor pressure.
However, one of the difficulties associated with these type systems is maintaining the phase equilibrium properties necessary to deliver the gas phase at a desired flow rate. The temperature and pressure of the compressed gas-liquid system in the bulk delivery vessel changes due to the varying gas flow rate withdrawn from the vessel. This phenomenon arises from the fact that under practical conditions the heat of evaporation utilized is not compensated by external heat. In other words, the compressed gas-liquid system contained in the bulk vessel can cool down significantly, affecting the conditions of the gas-liquid phase equilibrium, and hence the flow rate of gas conveyed to the point of use.
Further, withdrawal of vapor phase ESG at a high flow rate may entrain liquid droplets of the gas, thereby having a deleterious effect on the process and apparatus. For example, in the case of HCl, condensation occurs by the Joule-Thompson effect (see, Joule-Thompson Expansion and Corrosion in HCl System, Solid State Technology, Jul. 1992, pp. 53-57). Liquid HCl is more corrosive than its vapor form. Likewise, for the majority of chemicals listed above in Table 1, the liquid forms thereof are more corrosive than their respective vapor forms. Thus, condensation of these materials in the gas delivery system can lead to corrosion, which is harmful to the components of the gas delivery system. Moreover, the corrosion products can lead to contamination of the highly pure process gases. This contamination can have deleterious effects on the processes being run, and ultimately on the manufactured semiconductor devices.
The presence of liquid in the gas delivery system has also been determined to lead to inaccuracies in flow control. That is, the accumulation of liquid in various flow control devices can cause flow rate and pressure control problems as well as component failure, leading to misprocessing. One example of such behavior is the swelling of a valve seat by liquid chlorine, which causes the valve to become permanently closed. Such failure can necessitate shutdown of the process during replacement of the failed parts and subsequent leak checking. Extensive process downtime can result.
Thus, to meet the requirements of the semiconductor processing industry and to overcome the disadvantages of the related art, it is an object of the present invention to provide a novel system for controlled delivery of gases from a liquefied state which allows for accurate control of the flow rate.
It is a further object of the present invention to provide a method for delivery of gases from a bulk delivery vessel at a variable flow rate in a controlled manner.
It is another object of the present invention to provide a system wherein pressure controls are employed to monitor and adjust the energy input delivered to the bulk source vessel.
It is yet another object of the present invention to provide a system having an energy transfer device disposed to concentrate the energy to an area where it is needed.
Other objects and aspects of the present invention will become apparent to one of ordinary skill in the art upon review of the specification, drawings and claims appended hereto.